Medical device that applies electrical stimulation to the spinal cord from inside the dura for treating back pain and other conditions

ABSTRACT

This invention provides an device for electrical stimulation of the spinal cord. The device has an electrode assembly with a sufficiently thin profile to be implanted between the pial surface of the spinal cord and the dura mater, and secured to the dura. Electrodes on the electrode assembly are directed towards the surface of the spinal cord, and connected through the dura to a signal generator located outside the dura. Following implantation, the subject is treated by transmitting electrical signals from the signal generator through the leads to the electrodes, stimulating the subject&#39;s spinal cord.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/118,151, filed on Aug. 30, 2018 (now allowed); which is acontinuation of U.S. patent application Ser. No. 15/437,803, filed onFeb. 21, 2017, issued as U.S. Pat. No. 10,071,240; which is acontinuation-in-part of U.S. patent application Ser. No. 15/004,515,filed on Jan. 22, 2016, issued as U.S. Pat. No. 9,572,976; which is adivisional of U.S. Ser. No. 14/375,781, filed on Jul. 30, 2014, issuedas U.S. Pat. No. 9,254,379; which is the U.S. National Stage ofinternational Application PCT/US2013/23912, filed Jan. 30, 2013, whichwas published as WO 2013/116377 on Aug. 8, 2013; which claims thepriority benefit of U.S. provisional application 61/592,515, filed Jan.30, 2012; and U.S. provisional application 61/592,520 filed Jan. 30,2012.

U.S. patent application Ser. No. 15/437,803, is also acontinuation-in-part of U.S. patent application Ser. No. 15/267,765,filed Sep. 16, 2016 (abandoned); which is a continuation of U.S. patentapplication Ser. No. 14/821,540, filed on Aug. 7, 2015, issued as U.S.Pat. No. 9,486,621; which is a divisional of U.S. patent applicationSer. No. 13/885,157, filed on Jan. 6, 2014, issued as U.S. Pat. No.9,364,660; which is the U.S. National Stage of International ApplicationPCT/US2011/060462, filed on Nov. 11, 2011; which claims the benefit ofU.S. Provisional Application No. 61/412,651, filed Nov. 11, 2010.

U.S. patent application Ser. No. 15/437,803, is also acontinuation-in-part of U.S. patent application Ser. No. 15/191,214,filed Jun. 23, 2016, issued as U.S. Pat. No. 9,950,165; which is acontinuation of U.S. application Ser. No. 14/375,785, filed Jul. 30,2014, issued as U.S. Pat. No. 9,403,008; which is the U.S. NationalStage of International Application PCT/US2013/023897, filed Jan. 30,2013, which was published as WO 2013/116368 on Aug. 8, 2013; whichclaims the priority benefit of U.S. provisional application 61/592,520filed Jan. 30, 2012.

U.S. patent application Ser. No. 15/437,803, is also acontinuation-in-part of U.S. patent application Ser. No. 14/916,892,filed Mar. 4, 2016 (abandoned); which is the U.S. National Stage ofPCT/US2014/054243, filed Sep. 5, 2014, and published as WO/2015/035135on Mar. 12, 2015; which claims the priority benefit of U.S. provisionalapplication 61/874,340, filed Sep. 5, 2013.

All the aforelisted priority applications are hereby incorporated hereinby reference in their entireties for all purposes.

FIELD OF THE INVENTION

The invention relates generally to the field of medical devices and painmanagement. In particular, it provides electrode arrays and supportstructures for electrical stimulation of the spinal cord.

BACKGROUND

Chronic pain is an often unbearable sequelae of spinal cord injury ordisease. It can interfere with the basic activities, effectiverehabilitation, and quality of life of the patient. The prevalence ofpain in patients with spinal cord injury is high: in some studiesranging from about 62% to 84% of patients. Back pain is also a featureof other injuries and conditions. For example, postural abnormalitiesand increased muscle tone in Parkinson's disease may cause back pain,where the prevalence can be as high as 74%. Other conditions associatedwith back pain include disc rupture, congestive heart failure andosteoarthritis.

Because back pain is often intractable within the current spectrum ofclinical modalities, new technology is needed for pain management.

SUMMARY OF THE INVENTION

This application discloses various components of a technological systemthat can be used for stimulation of the spinal cord for the purpose oftreating back pain.

A plurality of electrodes is arrayed on a backing that conforms to thespinal cord. The electrodes can be configured for accommodating movementwhile stimulating the spinal cord of a subject, as part of a spinal cordstimulation apparatus. The floating electrodes are flexibly mounted tothe substrate such that when the electrode array is implanted into thesubject, individual electrodes float or move resiliently relative to thesubstrate to an extent sufficient to accommodate pulsations of thesurface of the spinal cord within the dura.

The electrode array can be maintained on the spinal cord at a chosenlocation by way of a spring or support structure that is anchored to ananatomical structure outside the spinal cord itself, but near the siteof implantation in the spine. Suitable anchoring structures include thevertebrae and the dura. Secured in this fashion, the compliant supportstructure maintains a gentle pressure of the electrode array against thespinal cord so as to stay in electrical contact but minimize the risk ofinjury or inflammation. The device can accommodate movement of thespinal cord laterally, transversely, and in a caudal-rostral fashion sothat the electrode array remains in place.

An aspect of the invention is an implantable device for stimulating thespinal cord of a subject. In general terms, the device has an array ofelectrodes configured to conform to a region of the spinal cord in thesubject such that the electrodes directly contact the spinal cord; and asupport structure configured for securing to an anatomical structureoutside the spinal cord and configured to urge the array towards thespinal cord so as to maintain contact of the electrodes in the array onthe spinal cord. Anatomical structures “outside the spinal cord” arebeyond the cord and the dentate ligaments, but preferably within thespine and supporting tissues, and include the vertebrae and the dura ofthe spinal canal.

The support structure may be configured for securing to the dura. Forexample, the support structure may have a first flexible member thatextends laterally towards and engages the left margin of the dura, and asecond flexible member that extends laterally towards and engages theright margin of the dura. Optionally, the first flexible member alsoengages the left dentate ligament, and the second flexible memberengages the right dentate ligament.

Alternatively or in addition, the support structure is configured forsecuring outside the dura: for example, to any one, two or more than twopositions on one or more vertebrae in the subject. Exemplary is a strapbridging the lamina of a single vertebra The support structure may alsohave a cuff configured to engage the dura at or near an access siteduring implantation of the device into the subject, thereby sealing theaccess site, and one or more electrical leads that pass from the arrayedelectrodes through the cuff to provide electrical power from outside thedura for stimulating the spinal cord.

An exemplary device for securing to a vertebra has the followingcomponents: (a) an array of electrodes configured to contact the spinalcord; (b) a deformable support structure configured to urge theelectrodes of the array into contact with the spinal cord duringmovement of the spinal cord within the dura; (c) one or more connectingmembers configured to pass from the support structure out through thedura; and (d) an attachment portion configured for securing theconnecting members to a vertebra. The support structure can beconfigured to maintain pressure of the array upon the spinal cord withina desired range while accommodating changes in position of the arrayrelative to the vertebra that result from movement of the subject inwhich it is implanted. The deformable support structure is compliant soas to be compressible during implantation, and has a spring action thaturges the structure in a direction that is substantially opposite of thedirection by which it was compressed. By gently compressing thestructure against the spinal cord during implantation, the device willcontinue to urge the electrode array towards the spinal cord once it isaffixed to a vertebra.

The support structure can comprise one or more flexible loops betweenthe array and the connecting member with one or more of the followingcharacteristics in any combination: the loops can be orientedsubstantially parallel with the spinal cord when the array of electrodesis in contact with the spinal cord; the loops can be oriented at anangle traversing the spinal cord when the array of electrodes is incontact with the spinal cord, thereby being in a position to accommodateboth transverse and lateral movement of the spinal cord; the loops canextend horizontally beyond the electrodes on the array; or the loops canconstitute or contain one or more electrical leads configured to supplyelectrical stimulation to the electrodes from a source outside thespinal cord.

The electrodes can be arrayed on a compliant backing that furthercomprises semi-rigid extensions in rostral and/or caudal directionsand/or lateral directions that increase surface area of the backing incontact with the spinal cord. Thus, the electrodes are disposed on aflexible substrate configured to conform to the spinal cord, wherein thearray has opposed axial ends along the spinal cord, and wherein thesubstrate extends sufficiently beyond both opposed axial ends so as toinhibit lifting of any electrodes of the array when the array moves withspinal cord physiological movement within the spinal canal along an axisof the spinal cord. In this configuration, the device may accommodate atotal rostral-caudal motion of about 2 cm without lift-off at either endof the backing.

The attachment portion can contain or be configured for securing to astrap, which in turn is secured to lamina of a vertebra of a subject soas to bridge the lamina. The device may also have a cuff skirting aroundconnecting member and configured to be joined with the dura at or nearan access site during implantation of the device into the subject,thereby closing the access site; and a scaffold portion attached to thevertical portion between the cuff portion and the spring portion,configured so as to be positioned beneath the access site after theaccess site is closed.

The connecting members may have one or more electrical leads configuredto supply electrical stimulation to the electrodes from a source outsidethe spinal cord. Such electrical leads can have a first lead portionextending from the attachment portion to the support and a second leadportion extending from the attachment portion to a stimulation signalgenerator, wherein the first lead portion is coupled to the second leadportion at a connector having a first connector portion mounted to theattachment portion, the first lead portion configured to be moreresistant to failure than the second lead portion. There may be anelectrical connector at or near the position where the device exits thedura, whereby electrical leads passing from the electrodes through theconnecting members to the connector may be electrically and reversiblyconnected to a power source.

Another aspect of the invention is a strap structured for securing tolamina of a vertebra of a subject so as to bridge the lamina, the strapbeing configured to receive and support the attachment portion of adevice, thereby maintaining pressure of the array of the device upon thespinal cord of a subject in which the device is implanted such that thepressure is maintained within a specified range. An implantable deviceof this invention and the strap may be manufactured, marketed, orsupplied separately or together in kit form.

Another aspect of the invention is an apparatus configured to receiveand install an implantable device of the invention in a subject at aposition wherefrom pain experienced by the subject can be relieved. Theapparatus has a holding member configured to receive and reversiblysecure the implantable device while it is being implanted into a subjectin need thereof, and a retractable measuring rod configured so that thespring portion or support structure of the device can be positioned andinstalled at a measured distance away from the spinal cord in thesubject such that the array of electrodes is urged upon the spinal cordwithin a desired or predetermined pressure range. The device and theapparatus can be manufactured, marketed, or supplied separately, ortogether as a combination.

Another aspect of the invention is a method for implanting a spinal cordstimulation device. The method comprises accessing the spinal cordthrough a surrounding dura of a spinal canal; positioning the spinalcord at a desired location within the spinal canal; placing an array ofelectrodes in contact with the spinal cord; coupling a deformablesupport structure between the array and the dura of the spinal canal sothat engagement between the electrodes and the spinal cord remainswithin a desired range as the spinal cord moves within the spinal canalfrom the desired location throughout a physiological movement range; andsealing the array and support within the spinal canal.

The device may be secured to a vertebra by creating an incision in thedura over the dorsal aspect of the spinal canal of the subject;positioning the arrayed electrodes over the dorsal spinal cord at alocation that is essentially symmetrical between the left and rightdorsal root entry zones; lowering the support structure towards thespinal cord so as to compress the spring portion and engage theelectrodes with the spinal cord within a desired pressure range; closingthe incision around the connecting members; and securing the attachmentportion to a vertebra of the subject. This may comprise loading thedevice on an installing apparatus of this invention, positioning andlowering the device onto the spinal cord by manipulating the apparatus,detaching and removing the apparatus from the device following step (e),and closing the incision around the connecting members once theapparatus has been removed.

The device may be secured to the dura by excising the dura over thedorsal aspect of the spinal canal of the subject; compressing the springor support structure; positioning the compressed device such that thearrayed electrodes engages a region of the spinal cord that was exposed;allowing the device to expand so that the array is urged towards thespinal cord; and closing the dura.

Another aspect of the invention is a method and a device for use instimulating a spinal cord by delivering an electrical stimulus to atargeted region of the spinal cord by way of a device according to thisinvention. The electrical stimulus can comprise a pattern of electricalpulses or signals. The stimulus is applied so as to inhibit sensation ofpain by the subject; or to inhibit symptoms of Parkinson's disease,spinal cord injury, or congestive heart failure in the subject.

Further embodiments of the invention will be apparent from thedescription that follows.

DRAWINGS

FIG. 1A is a cross-sectional view of a device configured to secure anelectrode array to a vertebra. The electrode bearing portion 11 is beingpositioned on the surface of the spinal cord 21 using a devicepositioning apparatus (DPA) 31. The dural cuff 14 is secured against theconnecting member 15 containing the lead 16. In FIG. 1B, the spacing rod33 is retracted, and the dura 23 is sutured to the dural cuff 14. InFIG. 1C, the device is secured to a strap 41 which in turn is secured tothe vertebral lamina 27 a and 27 b. FIG. 1D, the device positioningapparatus 31 has been disengaged from the device and removed.

FIGS. 2A to 2D are side (longitudinal) views of the device positioned onthe spinal cord and secured to a vertebra. FIGS. 3A to 3D are top-downviews of the device positioned on the spinal cord and secured to avertebra.

FIGS. 4A to 4E are side view illustrations showing a step-wise procedureby which the device may be installed into an operative position on thespinal cord. FIGS. 5A to 5C show the same procedure from a top downview. FIGS. 6A to 6D are magnified lateral views showing details of howthe device is positioned to optimize pressure of the electrode bearingportion on the spinal cord.

FIG. 7A shows an oblique view of a prototype device configured forsecuring to the inner margin of the dura. An electrode bearing portion11 is configured to conform to a surface of the spinal cord. Uponimplantation, extensions 17 of the backing project outwards from thespinal cord toward the support structure 10, where there are pins 18 forengaging the dura. FIG. 7B shows the device in cross-section, afterimplantation against the spinal cord 21 and being secured to the dura23. FIG. 7C is a detail showing suturing of the dural cuff 14 to thedura 23. FIG. 7D is a detail showing a securing pin 18 engaged in theinside of the dura 23.

FIG. 8A is a view of the device depicted in FIG. 7A, placed onto thespinal cord, viewed from above the dorsal surface of the spinal cord towhich the electrode array has been secured. FIG. 8B is a side view ofthe device where the spinal cord 21 is shown longitudinally, and thedura 23 has been cut away.

FIGS. 9A, 9B, 9C, and 9D show steps whereby the device may be implantedthrough a dural incision. The pins 18 on the support structure 10 engagethe dura 23, and the electrode bearing portion 11 conforms to the spinalcord 21 so that the electrodes may have direct contact.

FIG. 10A show an electrode array configured to be clamped to the dentateligament 28 on each side of the spinal cord. FIG. 10B shows a detail ofa clip 19 that affixes an extension 17 of the array to the dentateligament 28.

FIG. 11A is a schematic depiction of an electrode in cross-section,extending from the backing upon which it is arrayed. FIG. 11B showselectrodes arrayed in the backing so as to provide a degree of mobility.

FIGS. 12A and 12B are a drawing and a spinal cord image showingcalculation of the arc length between dorsal-root entry zones. FIGS. 13Aand 13B are images of a spinal cord showing movement of a spinal cordbetween neutral and flexed positions. The images can be used to measurespinal cord contraction and expansion along the rostral-caudal axis.

FIG. 14 illustrates a working prototype in which an array backing issecured to a transverse strap by way of a compliant (spring-like)structure in the form of a triple loop (loop area z 160 mm²). Whenmounted on a spinal cord stimulator apparatus, the prototypeaccommodated a total rostral-caudal motion of 2 cm without lift-off ofeither end of the backing.

FIG. 15 provides an overall view of a spinal cord stimulation apparatus.Included are an electrode bearing portion 11, a spring portion 12, astrap 41 for securing to a vertebra in the subject, an electricalconnector 82, and an external component 84, which is a signal generatorthat comprises a power source and electronics for controlling theelectrical stimuli.

DETAILED DESCRIPTION

One of the factors that limits spinal cord stimulation as an effectivetreatment of intractable back and leg pain is the inability of thestandard devices to selectively modulate the targeted neural fibers.Commercially available devices can inadvertently stimulate neighboringnon-targeted structures, such as the dorsal nerve rootlets. This is dueto the shunting effects of a layer of highly conductive cerebrospinalfluid located between the epidural electrodes and the spinal cordsurface. Shunting of the electrical current produced by the electrodeaway from the target treatment site limits therapeutic efficacy in up tohalf of all patients implanted with a standard epidural stimulator. SeeEldabe et al., Neuromodulation 13 201-209, 2010.

This invention provides a technology whereby electrode arrays can bemore reliably positioned in contact with the spinal cord. Gentlepressure is maintained using a spring or support structure anchored toan anatomical feature or structure outside the spinal cord. Suitableanchor points include anatomical structures at the margins of the spinalcanal (particularly the inner wall of the dura), and immediately outsidethe spinal canal (exemplified by the vertebrae).

FIG. 15 shows an embodiment of the securing system of this invention aspart of a spinal cord stimulation apparatus. An electrode bearingportion 11 having an array of electrodes is configured to conform to thespinal cord of a subject (not shown) who is in need of spinal cordstimulation. A spring portion 12 comprising flexible loops is configuredto urge the electrode bearing portion 11 against the spinal cord. Adural cuff 14 is configured for joining to the dura, sealing the springportion 12 within the intradural space. A connecting member 15 (carryingelectrical leads connected with electrodes in the electrode bearingportion 11) exits the dura, and is configured for attachment to a strap41, which is used to secure the device to a vertebra in the subject. Afirst lead portion 81 has an electrical connector 82 which connects to asecond lead portion 83 (a relay lead), which in turn is connected to anexternal component 84. The first lead portion 81 is generally moreresistant to failure than the second lead portion 83, which can bedetached at the electrical connector 82 and replaced. The externalcomponent 84 comprises a power source and electronics for controllingthe electrical stimuli.

The electrode array and support structures of this and relatedinventions and their various components are being commercially developedunder the mark “I-Patch”.

Rationale

The neural mechanisms that mediate the clinical effects of SCS arecomplex and likely involve activation of multiple ascending anddescending neural pathways within the spinal cord. In general,electrical stimulation will evoke sensory perceptions in the painfularea of the body in order for the treatment to be effective. Toaccomplish this, the region within the dorsal column of the spinal cordthat contains axons that are functionally related to the painful bodyarea must be activated. Dorsal column axons are somatotopicallyorganized, meaning that the axons that are functionally related to aparticular body area are positioned in close proximity to each other,and there is an orderly anatomical pattern of organization within thespinal cord for the different groups of axons linked to different bodyareas. In the cervical spinal cord, for example, dorsal column axonsfunctionally linked to the back region may be relatively close to themidline of the spinal cord, and axons linked to the arms are positionedrelatively more laterally.

The spinal cord is axially cylindrical, and positioned centrally withinthe spinal canal. The spinal canal is lined by a dural membrane andcontains cerebrospinal fluid (CSF) that surrounds the spinal cord andfills the region between the outside surface of the spinal cord and theinside surface of the dural membrane. This CSF-filled space plays acritical role in normal spinal cord biomechanics and is an importantfactor that should be considered when performing spinal surgery. Duringnormal movements, such as flexion and extension of the body, the spinalcord moves within the spinal canal, altering its position relative tothe dural lining of the spinal canal. The volume of CSF surrounding thespinal cord serves as a frictionless buffer during these movements. Insome pathological conditions (e.g. tethered cord syndrome) this normalmotion of the spinal cord is impeded by tissue attachments bridging thespace between the spinal cord and the dural lining, resulting indysfunction of the spinal cord. In other pathological conditions, atissue barrier forms within the spinal canal (e.g. following trauma orinfection) that disrupts the normal flow of CSF over the surface of thespinal cord. In these settings, CSF may accumulate within the substanceof the spinal cord to form a syrinx and cause neurological dysfunction.

The technology in this disclosure addresses deficiencies of previouslyavailable SCS devices by incorporating at least some of the followingdesign features:

-   -   the electrical stimuli are delivered directly to the spinal        cord;    -   a dense array of electrode contacts enables delivery of highly        localized, spatio-temporally synchronized and positionally        selective electrical stimuli to any targeted sub-region of the        spinal cord;    -   the device does not mechanically tether or form a physical        connection between the spinal cord and dura that significantly        alters the natural support and flexibility provided by the        dentate ligaments;    -   the implantable electrode assembly has an ultra-thin physical        profile that does not obstruct or alter CSF flow patterns around        the spinal cord;    -   the contact forces between the device and the spinal cord are        stable and unvarying, and hence patient movement does not affect        these contact properties, which results in optimal electrical        coupling between electrode contacts and spinal cord tissue;    -   the surgical procedure used to implant the device is well        established and safe, and does not substantially increase the        risk of complications; and    -   the manufacturing cost of the devices provided in this        disclosure may be less than that for previously existing        devices.

Securing the Electrode Array to the Vertebrae

FIGS. 1A to 3D illustrate an example of the invention in which anelectrode array is secured to a vertebra of the subject. FIGS. 1A to 1Dare a cross sectional views. FIGS. 2A to 2D are side (longitudinal)views. FIGS. 3A to 3D are top-down views.

Referring to FIG. 1A, the spinal cord 21 in the spinal canal 22 issurrounded by the dura 23. Dorsal rootlets 24 carry sensory (afferent)fibers to the spinal cord. Dentate ligaments (not shown) suspend thespinal cord within the spinal canal. This is surrounded by a vertebra 26that been opened at the back (top) through which the surgeon may accessthe spinal cord.

The device comprises an electrode bearing portion 11 bearing an array ofelectrodes for contacting and conforming to the spinal cord 21, fromwhich to deliver an electrical stimulus. The device further comprises adeformable spring portion 12 in the form of a loop, which acts as asupport structure, urging the electrode bearing portion 11 into contactwith the spinal cord 21 so as to accommodate movement of the spinal cordwithin the spinal canal 22.

Referring to FIG. 1B, upon implantation into a patient, the electrodebearing portion 11 is placed on and conforms to the surface of thespinal cord 21. The electrode bearing portion 11 is kept in place by wayof the spring portion 12 that presses the array against the cord. Here,the spring portion 12 is depicted as a transverse loop. Alternatively,the loop runs longitudinally along the spinal cord or diagonally toaccommodate movement of the spinal cord in a caudal-rostral fashionthrough the vertebrae. Over top of the spring portion 12 is anattachment arm or scaffold 13, above which is a dural cuff 14 forsuturing to the dura 23 during closure. A connecting member 15containing electrical leads 16 passes up through the dural cuff 14 to anexternal power source (not shown) that provides a pattern of electricalpulses for stimulating the spinal cord.

Referring to FIG. 1C, following closure of the dura, the strap 41 isplaced with lower surfaces 42 a and 42 b in contact with the vertebrallamina 27 a and 27 b, where it may be permanently affixed by way ofsurgical screws (not shown, see FIGS. 3C and 3D below) or otherattachment means (not shown). The vertical connecting member 15 of thesupport device passes through and is secured to an opening 44 near themidpoint of the strap 41 between the two vertebral lamina 27 a and 27 b.The device may also comprise an attachment means (not shown) by whichthe connecting member 15 is secured to the strap 41 at a set distancefrom the spinal cord 21 so as to maintain pressure of the electrodebearing portion 11 against the spinal cord 21 within a desirable rangeof pressure. The electrode bearing portion 11 thereby maintains aposition wherefrom to stimulate the spinal cord 21 without losingcontact should the spinal cord move from its neutral position, withoutinjuring the spinal cord and surrounding tissues, and without provokingan inflammatory response.

FIGS. 1A to 1C also show a micromanipulator that is used as a devicepositioning apparatus (DPA) 31. The surgeon may use the DPA to installthe device into a subject at a target site on the spinal cord. The DPAcomprises a holding member or handle 32 configured to receive andreversibly secure the implantable device while it is being implanted,and a measuring portion or spacing rod 33 with a lower surface 34configured to be placed upon the dorsal surface of the spinal cord 21during the procedure. The DPA is used to position the spring portion orsupport structure 12 of the device at a measured distance away from thespinal cord 21 such that the electrode bearing portion 11 is urgedagainst the spinal cord 21 within a desired pressure range.

The spacing rod 33 is used to calibrate the distance for lowering theelectrode bearing portion 11 onto the spinal cord 21. The spacing rod 33may be permanently attached to the DPA, or may be retractable. As shownin FIG. 1A, the spacing rod 33 passes through brackets 35 a and 35 b sothat the spacing rod 33 can be lowered to a measuring position, and thenraised by way of a handle 36 to a retracted position. The device may bedetachably secured to the DPA, for example, using a clamping arrangementor fungible connecting means such as a thin band or suture. The DPA isoperated to release the device once the device is secured at the targetlocation on the spinal cord, and then removed from the field. In FIG.1D, the device positioning apparatus 31 has been disengaged from thedevice and removed.

FIG. 2A is a side view showing the reflected edge 23 a of the dura as adashed line. The electrode bearing portion 11 is shown during initialpositioning on the spinal cord 21 with the dural cuff 14 secured againstthe connecting member 15 above the scaffold 13. The spacing rod 33 ofthe positioning apparatus 31 is in the down position in contact with thespinal cord 21. In FIG. 2B, the spacing rod 33 is moved to the upposition, and the dural cuff 14 is secured by suturing to the dura 23.In FIG. 2C, the titanium strap 41 is secured to the vertebral lamina(not shown), and the vertical connecting member 15 is secured to thestrap 41. In FIG. 2D, the device positioning apparatus 31 has beendisengaged and removed.

FIG. 3A is a top-down view showing the rostral 23 b and caudal 23 climit of the durotomy, with the electrode bearing portion 11, theloop-shaped spring portion 12, the scaffold 13, and the dural cuff 14,with the dural cuff secured against the vertical connecting member 15before implanting. The spacing rod 33 of the positioning apparatus is inthe down position. FIG. 3B shows the dural cuff 14 detached from thevertical connecting member 15 and sutured to the dura 23, with thespacing rod 33 moved to the up position. FIG. 3C shows the verticalconnecting member 15 of the device secured to an opening 44 at aroundthe horizontal midpoint of the strap 41, which in turn is secured to thelamina 27 a and 27 b by way of four surgical screws 43 a, 43 b, 43 c, 43d. In this illustration, the strap bifurcates on each side as itapproaches the lamina. In FIG. 3D, the device positioning apparatus hasbeen disengaged and removed.

Components of the Device

In more general terms, a spinal cord stimulating device of thisinvention may include any of the following features in any operativecombination.

For securing to a vertebra, the device comprises an electrode array (asdescribed in more detail below), and a spring portion or supportstructure that maintains contact of the electrodes in the array with thespinal cord. The spring portion or element is configured (by a choice ofshape, thickness, rigidity, and distance away from the array itself) toexert a pressure by the array on the spinal cord within a predeterminedor desired range upon implantation of the device into a subject. Therange of this pressure might be 0.1 mm Hg through 25 mm Hg. Variousmechanical spring shapes are suitable for this compliant element. Onesuch shape is one or more flexible loops that is attached to the upperstructure of the device on one side and to the array on another side.For economy of design and operation, the spring portion consists of orcontains the electrical leads supplying stimulation to the electrodes.

The support structure may integrally comprise an attachment portion forsecuring directly to a vertebra. Alternatively, it may comprise one ormore vertical connecting members configured for securing to a separatestrap that bridges the lamina. The strap may have any suitable shapethat spans between and secures to the lamina or other parts of thevertebra within a suitably confined volume. The strap may be packaged orprovided together with other components of the device in kit form, orsupplied separately.

Other possible components include a cuff portion attached to thevertical connecting member and configured to be joined with the dura ator near an access site during implantation of the device into thesubject, thereby closing the spinal canal. A scaffold portion may beattached to the vertical portion between the cuff portion and the springportion, configured so as to be positioned beneath the access site afterclosure. There may also be an electrical connector at or near theposition where the device exits the dura, whereby electrical leadspassing from the electrodes through the vertical member(s) to theconnector may be electrically and reversibly connected to a powersource.

Alternatively or in addition to the securing apparatus described above,the electrode array may also be provided with attachment arms that wrapat least part way around the spinal cord. For example, flexibleattachment arms may extend from either side of the electrode array, withthe attachment arms typically being formed at least in part of thesubstrate or backing material on which circuit components are mounted orformed. Further information about various wrap-around configurations isprovided in U.S. Pat. No. 9,364,660.

Apparatus for Positioning the Electrode Support Device DuringInstallation

As explained above in reference to FIGS. 1A to 1C, the devicepositioning apparatus (DPA) 31 assists the surgeon in placing thesupport structure with the electrode bearing portion precisely onto thetarget site on the spinal cord. The DPA is configured to receive andinstall the device in such a manner that the electrode array abuts andis urged against the dorsal surface of the spinal cord, and is anchoredto a nearby vertebra. The DPA comprises a holding member configured toreceive and reversibly secure the implantable device while it is beingimplanted, and a spacing rod 33 configured so that the spring portion 12or support structure of the device can be positioned and installed at ameasured distance away from the spinal cord 21 in the subject such thatthe electrode bearing portion 11 is urged upon the spinal cord within adesired pressure range.

The DPA can be configured as a rod-shaped hand held device. Duringsurgery, the device is connected to a malleable attachment arm (e.g.modified Greenberg) outside of the surgical cavity. The neurosurgeongrasps and positions the DPA using her left hand. The device assembly isreversibly attached to the end of the DPA. A heavy-gauge suture is runthrough eyelets on the DPA and looped around the exiting lead of thedevice. During the insertion procedure the suture is under sufficienttension (the sutures are secured to anchor points on the distal handleof the DPA) to grasp the lead securely. At the appropriate point in theprocedure, the suture is cut using microscissors to achieve a smooth andtechnically simple mechanical release of the device lead from the DPA.

The portion of the DPA rod close to the spinal cord has an acute angle.This angle serves to off-set the point of attachment between the DPA andthe device lead several centimeters away from the shaft of the DPA. Thisdesign feature serves two purposes: 1) it provides a clearlyline-of-sight field of view for the neurosurgeon than would be the casewith a straight DPA, and 2) it gives the neurosurgeon the physicalaccess to suture a majority of the device dural cuff circumference tothe spinal dura while the device is being held in the optimal positionby the DPA. After the device is secured by way of the strap to thelamina of the vertebra, it may not be technically feasible to surgicallyaccess much of the circumference of the dural patch. All but a smallportion of the dural cuff circumference is sutured closed before thestrap is placed, and the portion that remains to be sutured is well awayfrom the strap (i.e. a small portion of the cuff located on the side ofthe durotomy opening opposite to the strap).

As illustrated in FIG. 4A, the DPA 31 may also be provided with anaffixed or retractable distance measurement extension (DME) 37 ormeasuring rod. The purpose of the DME is to provide the neurosurgeonwith visual feedback to set the appropriate distance between theelectrode bearing portion of the device and the point at which the leadbundle penetrates the dural cuff during the device implantationprocedure. The DME may be gently curved, extending below and parallel tothe distal portion of the DPA (the end of which is attached to thedevice lead). The neurosurgeon is able to visualize the tip of the DMEand lowers the device assembly onto the spinal cord, compressing theintradural leads until the tip of the DME just comes into contact withthe electrode bearing portion of the device. The midline position of theDME does not interfere with the first portion of the subsequent duralcuff closure step, or damage the intradural leads. Once the device ispermanently secured to the strap and it is necessary to disengage andremove the DPA, the curved profile of the DME allows it to be safelyremoved from the intradural space with a simple rotational and upwardmovement of the DPA.

Procedure for Installation

General guidance for the surgeon in placing the device onto the spinalcord is as follows. The surgeon should have clear line-of-sightvisualization of the electrode bearing portion of the device as it ispositioned on the surface of the spinal cord. The distance between theelectrode bearing portion of the device and the point at which the leadsfuse into the dural cuff exit site can be set precisely during theoperation. This sets the tension of the malleable intradural leads atthe optimal force under conditions when the spinal cord is at its mostventrally displaced position within the spinal canal.

The surgical opening should provide access for forceps and a needledriver, around the entire circumference of the dural cuff. The deviceplacement and closure technique should be efficient, with no unnecessarysteps. The surgical technique can be designed in a manner that minimizesthe risk of injuring the spinal cord or damaging or displacing theelectrode array from its optimal location. A technically competentneurosurgeon should be able to complete the implantation proceduresafely and reliably.

FIGS. 4A to 4E are side views that illustrate a step-wise procedure bywhich the device may be installed into an operative position on thespinal cord. FIGS. 5A to 5C show the procedure from a top-down view. InFIG. 5A, the solid line labeled 23 a is the edge of reflected spinalcanal dura. The dashed line labeled 14 a is the caudal edge of the duralcuff. FIGS. 6A to 6D are magnified lateral views showing details of howthe device is positioned to optimize pressure of the electrode bearingportion on the spinal cord. In these illustrations, the securing strapis shown as a substantially rectangular shape and secured to each of thetwo vertebral lamina by way of two surgical screws on each side. Thedevice positioning apparatus (DPA) 31 is shown with an affixed(non-retractable) distance measurement extension (DME) 37.

The connecting member 15 of the electrode array support device issecured to the strap 41 at or near the midpoint between the sides thatspan the lamina at or near the rostral edge of the strap. This can bedone, for example, using sutures, small clamps, or an adhesive such assilicone. There may be a portion of the strap designed to accommodatethe lead and the suture or clamp. The components are configured so thatthe device may be positioned to press the electrode bearing portion 11on the spinal cord 21 at a desired pressure, and then secured in thisposition by way of the strap 41 or attachment portion.

The steps of the procedure are as follows.

-   -   Step 1: A multi-level laminectomy is performed.    -   Step 2: A mid-line durotomy is created of sufficient length.    -   Step 3: The device is loaded onto the DPA. This requires looping        a suture through the DPA eyelets and around the lead, and then        securing the suture on the DPA handle using sufficient tension.    -   Step 4: The DPA is attached to a malleable retractor arm        (modified Greenberg), and the surgeon uses his left hand to move        the DPA into position. The retractor is kept on the flexible        setting (maximal malleability) until the surgeon has the DPA and        attached device in the desired position. Once the device is in        the optimal position, an assistant secures the retractor arm to        achieve rigid fixation of the DPA and device.    -   Step 5: Using forceps held in his or her right hand, the surgeon        grasps the caudal portion of the device dural cuff and reflects        it upward to allow clear visualization of the electrode bearing        portion of the device.    -   Step 6: The DPA is adjusted so that the electrode bearing        portion of the device is positioned on the dorsal spinal cord        symmetrically between the left and right dorsal root entry        zones.    -   Step 7: The DPA is then lowered further towards the spinal cord        causing the intradural leads to bow outward. This compression        maneuver is continued until the DME makes gentle contact with        the electrode bearing portion of the device. This is then the        optimal position for the device assembly.    -   Step 8: The assistant adjusts the malleable retractor setting to        lock the retractor (and the attached DPA) into the optimal        position.    -   Step 9: The surgeon sutures the accessible edge of the dural        cuff to the spinal canal dura. The surgeon's line of sight at        this stage in the procedure is from a caudal vantage point.        Approximately 80% of the circumference of the dural path is        sutured to the spinal canal dura from this surgical angle. The        only portion of the dural cuff that is not sutured closed is the        portion obscured by the section of the DPA that extends rostral        from its point of fixation with the device lead.    -   Step 10: The strap is secured to the device lead and then        secured to the left and right edges of the laminectomy defect        using bone screws. This step secures the device assembly into        its final position relative to the spinal canal.    -   Step 11: The suture securing the DPA to the device assembly is        cut, disconnecting the DPA from the device lead.    -   Step 12: The surgeon holds the DPA as an assistant changes the        retractor arm to the malleable setting. The surgeon then gently        rotates and lifts the DPA out of the surgical cavity. Care is        taken to remove the DPA from the subdural space without        impacting the device assembly or damaging the intradural leads.    -   Step 13: The surgeon shifts his line of site in order to        optimally visualize the rostral portion of the dural cuff. This        portion of the dural cuff is sutured to the spinal canal dura.    -   Step 14: The remainder of the wound closure is carried out in a        manner similar to that used for a standard spinal cord        stimulator placement operation.

In Steps 7 and 8, the surgeon has an opportunity to optimize thepressure of the array on the spinal cord so that the device accommodatesmovement of the spinal cord, but without injuring the spinal cord orcausing inflammation. Depending on the nature of the device and thejudgment of the surgeon, a suitable pressure could be about 0.1 mm Hg to25 mm Hg. The upper limit (25 mm Hg) is about half of a typical lowrange human diastolic blood pressure, so that blood flow through surfacevessels on the spinal cord would not be choked off by the pressureapplied to them by the electrode-bearing surface.

Securing the Electrode Array to the Dura

As an alternative to securing the electrode array to the vertebra, itcan be secured to the margins of the dura. There are several advantagesof this approach. Surgical implantation of the device may be lesscumbersome for some patients. A simple, one-piece device is placed onthe surface of the spinal cord, and allowed to expand to engage thedura. No clips or screws are required, and lead manipulation isminimized. Furthermore, the device can be extracted or repositioned moreeasily. By compressing the device laterally, it disengages from the duraand can be removed from the patient or reengaged elsewhere. This may bean easier surgical procedure that is less likely to cause damage thanusing clips, clamps, tabs, fusions, or other attachment means andprocedures.

This section describes and illustrates a device of this nature andexplains how it may be implanted onto the spinal cord of the patient.The features shown in the illustration are not meant to be limitingunless explicitly or otherwise required.

FIG. 7A shows an oblique view of the device before implantation onto thespinal cord of a patient. An electrode bearing portion 11 bears an arrayof electrodes that face downwards, thereby being configured for directcommunication with the spinal cord 21 so that the electrodes may provideelectrical stimulation to the region of the cord upon which it is laid.The electrode bearing portion 11 has extensions 17 that extend laterallyin both directions, so as to join with the support structure 10. Theplane of the electrode array also typically comprises electroniccircuitry to create or convey a pattern of electrical stimulation (notshown). Power and control signals may come to the electronic circuitrythrough a wire connection that passes outside the device, or it may bereceived through an antenna in the device by wireless transmission.

The support structure 10 shown here is in the form of a half-ovalscaffold (HOS). The function is to secure the array 11 to the left andright lateral margins of the dura upon implantation into the spinalcanal. The HOS is slightly compressed during surgical insertion When theHOS is released, small securing pins 18 on each side attach to sites onthe left and right lateral martins of the spinal canal dura. Theelectrode bearing portion 11 is suspended between each side of thesupport structure 10 by way of extensions 17 of the backing materialthat extend laterally from the electrode bearing portion 11. Because oftheir positioning during implantation, the lateral extensions 17 may bereferred to as “artificial dentate ligaments”.

The embodiment shown here further comprises a dural cuff 14. This can beused to fix the device to a third point of the dura: specifically, thesite of the dorsal durotomy closure. Fixation of the cuff duringimplantation can achieve a water-tight dural closure around theconnecting member 15 containing electrical leads, as it exits the dura.

FIG. 7B shows the prototype device in cross-section as it would beimplanted onto a spinal cord 21 (shown with white matter 21 a and graymatter 21 b). The electrode-bearing portion 11 directly contacts thespinal cord, providing a medium through which electric stimulation maybe provided. The electrodes are imbedded in a compliant backing of anelastic or pliable backing material so that the array may conform to thespinal cord 21 at a region where the clinician desires to provide anelectrical stimulus. The electrodes are exposed downwards to engage thespinal cord such that all or most of the electrodes remain in contactwith the spinal cord providing a conduit for imparting the electricalstimulus.

Situated above the electrode bearing portion 11 in this illustration isa support structure 10, shown here curved nominally in the shape of ahalf oval. The support structure is typically made of a material that ispliably more rigid than the backing for the electrodes. This enables thestructure to resiliently support the electrode array, and provide anoutward pressure (when laterally compressed) to urge the sides towardsthe dura, thereby engaging the inner margin. In this example, thebacking of the array assembly has extensions 17 (the artificial dentateligament) that extend towards the support structure 10 on each side,thus anchoring the array downwardly upon the spinal cord 21.

FIG. 7D provides a detail of the securing pin 18 engaging the dura 23 onthe left and right margins. FIG. 7C provides a detail of the thirdfixation point. The dural cuff 14 disposed above the support structure10 is sutured or otherwise fixed to the dura 23 at an incision point,such as may be generated by surgical insertion of the device. In thisillustration, a connecting member or sheath (not shown) passes throughthe dural cuff 14 and out of the spinal canal, carrying leads 16 to anexternal component that supplies power and control signals to electrodesin the electrode bearing portion 11.

FIG. 8A shows the implanted device in a top down view. The dura 23 isshown in transverse cross-section. The electrode bearing portion 11 isattached to the support structure 10 by way of the backing extensions17, with the connecting member 15 exiting the spinal canal upwards. FIG.8B shows a side view of the spinal cord 21 with the dura 23 intransverse cross-section. The electrode bearing portion 11 is maintainedin direct contact with the spinal cord 21 white matter by way of theextensions 17 attaching to the support structure 10 at or around thepoint where the pins 18 secure the support structure to the dura 23.

Installation of the Device

The device may be affixed to the dura by way of contact points thatadhere to or transverse the dura. Alternatively, the device can beconstructed in such a fashion that its sides span the width of thespinal canal, and/or maintain a gentle lateral pressure so as to securethe device in place.

FIGS. 9A to 9D show how the device may be implanted such that theelectrodes are put in contact with the spinal cord 21. In FIG. 9A, anincision is made in the dura 23 from the dorsal side that is largeenough for the compressed device to be introduced into the spinal canal.The device comprising the electrode bearing portion 11 and the supportstructure 10 is compressed inwardly by way of forceps 51 that arespecially designed to configure to the sides of the device and compressthe sides and hold the device in the compressed or smallerconfiguration. In FIG. 9B, the electrode array is passed through theincision and positioned over the exposed dorsal surface of the spinalcord. In FIG. 9C, closing tension on the forceps is reduced, allowingthe support structure 10 to expand laterally, thereby urging the duralsecuring pins 18 into the dura 23. In FIG. 9D, the dura 23 is closed,incorporating the device's dural cuff 14 into the closure. Each contactpoint with the dura may have a plurality of pins, a pad, a smooth orrough surface integral to the extension or support structure, or acontact feature is discrete and mounted onto the device, facing outwardstowards the dura. The pin or other contact feature may be configured toengage the dura without rupturing the surface, or it may pass into orthrough the dura, optionally to be secured on the other side with a cap.

Securing the Electrode Array to the Dentate Ligaments

Alternatively or in addition to securing the electrode array to thevertebra and/or the dura, the device may be secured to or otherwiseplaced in communication with the dentate ligaments. One normal functionof the dentate ligaments is to suspend the spinal cord within the spinalcanal in a physiologic manner that enables supple movement of it butdoes not cause chronic injury to the spinal cord from mechanicaltethering.

FIG. 10A shows an electrode array adapted for clamping to the dentateligaments. The device has an electrode bearing portion 11 supported by asupport structure in the form of a body 12 a which includes a flexiblesubstrate or backing, with the array configured to engage a dorsalportion of the spinal cord 21. Dentate ligament attachment features suchas flexible arms 17 a extend laterally from left and right sides, withthe arms optionally comprising the same substrate or backing materialfrom which the body is formed. The extensions are configured to beattached to left and right dentate ligaments 28 on either side of thetreatment region of the spinal cord to secure the electrode bearingportion 11 in engagement with the spinal cord. The attachment arms 17 amay be more elastic than the array backing, extending laterally from theelectrode array. The attachment arms 17 a may flair to a larger widthadjacent the ends opposite the array, or may have slightly raised grovesor texture at or near these ends to facilitate clipping, crimping, oradhesively bonding the arms to the dentate ligament. FIG. 10B shows adetail of the clip or tab 19 used to attach the arms 17 a to the dentateligament 28.

Electrode Design

FIG. 11A schematically illustrates an electrode projecting from aninterior surface of a backing or substrate. Therapeutic benefit may beenhanced by maximizing current densities in the targeted conductingtracts of the spinal cord itself, while minimizing the current densityshunted away by the CSF. In this embodiment, the electrodes are engagedagainst the surface of the spinal cord as shown, with a stand-off column62 extending between the exposed portion of the electrode 61 and theunderside of the electrode backing 64. This arrangement can support theimplant off the surface of the spinal cord by about 100 μm toaccommodate pulsation of the spinal cord 21. By insulating the surfaceof stand-off column 62, it is possible to minimize the shunting effectof the CSF, since the exposed portion of the electrode will be incontact only with the pial surface of the spinal cord 21, and not withthe CSF itself. Gentle inward pressure causes slight inward “dimpling”of the pial surface by the electrode. As a result, the active exposedsurface of the electrode is “sealed” by spinal cord tissue envelopingthe protruding portion of the contact. A small gap separates theelectrically inactive portions of the array, providing space into whichthe spinal cord tissue may expand and contract with cardiac pulsationcycles.

FIG. 11B schematically illustrates individual electrodes 61 flexiblymounted to a substrate or backing 64 by a soft resilient material 65 soas to allow the electrode to resiliently float or move radially and/orlaterally relative to the substrate by a distance that is at least aslarge as the pulsations of the surface of spinal cord 21. This movementof each electrode may inhibit sliding engagement of the electrodesagainst the surface of the spinal cord during pulsation or any othertype of spinal cord movement. In some implementations, the only parts ofthe array that directly engage the spinal cord are the electrodecontacts. These may serve as mechanical anchoring points for the device.They exert enough pressure to maintain good electrical contact with thesurface of the spinal cord. The pressure exerted should be generallyeven for all of the contacts, for example, by having electrodesprotruding slightly from contoured attachments arms (not shown). Thisplaces all contacts in the desired position in relation to the surfaceof the spinal cord. Outward and inward movements of the contacts (e.g.with pulsations and respirations) are accommodated by movements of thesemi-rigid attachment arms.

The bodies of the electrodes 61 extend through apertures 66 in substrateor backing 64, with the substrate being pliable and having elasticityappropriate to supporting thin film circuit components. A softelastomeric material 65 spans the apertures from the backing 64 to theelectrodes 61, with the elastomeric material here comprising a sheet ofmaterial adhered to the outer surface of the substrate. Alternatively,the electrodes 61 may be supported relative to each other and thesubstrate with a soft elastomeric material spanning directly between theelectrode and the walls of the aperture. Alternatively, the electrodesmay be supported relative to each other and the substrate with a softelastomeric material spanning directly between the electrode and wallsof the aperture. Flexible conductors (not shown) may extend between thesubstrate and electrode bodies within or outside the elastic materialwith these conductors optionally being serpentine, having loops, orotherwise configured to accommodate movement of each electrode bodyrelative to the substrate.

As shown in the figure, the electrode bodies 234 extend throughapertures 238 in substrate 230, with the substrate being pliable andhaving elasticity appropriate to supporting thin film circuitcomponents. A soft elastomeric material 236 spans the apertures fromsubstrate 230 to the electrode bodies, with the elastomeric materialhere comprising a sheet of material adhered to the outer surface of thesubstrate. Alternatively, the electrodes may be supported relative toeach other and the substrate with a soft elastomeric material spanningdirectly between the electrode and walls of the aperture. Alternatively,the resilient material may form column 220. Flexible conductors (notshown) may extend between the substrate and electrode bodies within oroutside the elastic material with these conductors optionally beingserpentine, having loops, or otherwise configured to accommodatemovement of each electrode body relative to the substrate.

Other Features of the Electrode Array

A support device according to this invention presents an array ofelectrodes configured to conform to a region of the spinal cord suchthat the electrodes directly contact the spinal cord. Suitable electrodearrays are described below and in WO 2012/065125. A compliant backing istypically used, and is reshaped with a curvature to lie on top of thedorsal aspect of the spinal cord so that the electrodes across the arrayare in contact with the dorsal pial surface of the spinal cord surface.

An implantable device according to this invention comprises a pluralityof electrodes for placing in direct contact or electrical communicationwith the pial surface and underlying white matter of the spinal cord,within the spinal canal. The electrodes may be arrayed on a pliablebackground, constructed of a material and in a shape that allows it tobe conformed directly to the spinal cord. The plurality of electrodesmay comprise at least 10, at least 20, at least 30, or at least 50electrodes. They may be arrayed on the backing in a grid, a rectilinearpattern, or any other arrangement that is effective. All of theelectrodes may be supplied with stimulating power through a common lead.Alternatively, the electrodes may be attached singly or in groups toseparate leads so that each electrode or electrode group can provide thespinal cord with a separate stimulus as programmed by a central controlunit.

Array Design

The electrode bearing portion (EBP) of the array is generally structuredto conform to the dorsal surface of the spinal cord and maintain each ofthe electrodes in contact so as to deliver an electrical signal.Suitable parameters for the array may be drawn from Example 1, below.The array may be provided in a range of stock sizes. Alternatively, itcan be custom manufactured according to the anatomy and treatmentobjectives for each patient. The exact anatomical dimensions of a givenpatient's dorsal spinal cord can be determined from a pre-operative MRIstudy. The EBP is manufactured to be semi-rigid and have a fixedcurvature that is appropriate for a given patient's spinal cord.

The array will be held in place by forces exerted by a plurality ofleads spanning the space between the EBP on the spinal cord surface andthe dura. The direction and magnitude of forces exerted on the EBP willvary significantly as the spinal cord moves within the spinal canal. Insome extreme spinal cord positions there will be a tendency for the leadforces to cause the EBP to lift or rock out of position. This effect canbe reduced by adding non-lead bearing physical extensions to the EBP.Extensions of semi-rigid silicon in both the rostral and caudaldirections make the EBP less likely to lift off of the spinal cordsurface when the cord is displaced. Similarly, laterally positionedextensions help prevent lateral slippage. These extensions may bereferred to as “outriggers”. The configuration of the electrodes on thearray and the stimulus patterns used to energize them may be selected tooptimize the distribution of electrical current density within thetarget tissues of the spinal cord, while minimizing the spread of saidelectrical current density into non-targeted tissues, for example thedorsal root entry zones.

Electrical Leads

Where the device receives power or control signals from an externalsource by way of wire leads, the leads pass through the dura to be inelectrical communication with the circuit and the electrodes. Byproviding the leads with a cuff, flange or other feature for attachmentto the dura where the leads pass through, a fixation point for thedevice is created by attaching or sealing the feature to the dura. Afterpassing into the spinal canal, the leads may trace a path along theextensions or support structure (either internally or on either side),and then continue to the circuit or electrode array.

Lead segments between the electrode array and the dura may be configuredto serve at least two functions. One is to conduct electrical signals;the other is to exert the desired physical forces on the EBP to maintainits optimal position on the mobile spinal cord: specifically, a gentle,stable pressure on the EBP for a wide range of spinal cord positions.Long, looping leads can be used that are oriented predominantly in therostro-caudal plane, but with a slightly oblique orientation to deliversome components of force in directions that help prevent left-rightmigration of the EBP along the surface of the spinal cord, or torqueingof it about the axial direction. These leads flex and extend as the cordmoves within the spinal cord. A preferred material for the leads is ahighly malleable, braided lead made of a super-alloy (MP35N/silver-coreDFT wire stranded cable).

If a lead were to break, it would be better for this to occur in such away that the repair procedure does not require that the intraduralportion of the device be replaced. As shown in FIG. 15, a design featureto address this issue is to place a connector 82 on a first lead portion81 located close to where the first lead portion exits the dura. A relayor second lead portion 82 is then used from that point to the stimulusdelivery unit located in the external component 84. The overall systemis thereby configured so that, if a lead break were to occur, it wouldbe much more likely to occur in the relay than in the intradural portionof the device.

Device Components and Commercial Distribution

A device according to this invention may be part of a system that alsocomprises external components: particularly a power supply, and acontrol unit that sends control signals to the circuitry or electrodeson the implanted device. This is shown in FIG. 15. The externalcomponent 84 may provide both a power source and electronics forcontrolling the electrical stimuli. There may be a microprocessor orother suitable controller that is programmed to shape the electricalstimuli into one or more particular patterns, and to regulate thefrequency of an alternating current. The external component of thedevice may be configured to receive operator input regarding stimuluspattern selection and/or amplitude and frequency. Alternatively or inaddition, the external component may also be configured to receivefeedback data and to adjust the pattern and/or amplitude and frequencyto improve the effect perceived by the patent.

Optionally, the circuitry controlling the stimulus supplied by theelectrodes may be built into the same backing as the electrodes. Powerand control signals can be provided to the circuitry and the electrodesby electrical leads that pass in and out through the dura.Alternatively, the device may have a receiving means such as an antennathrough which to receive power and control signals wirelessly from anexternal source.

The device and technology of this invention can be used for diagnostic,therapeutic, and research purposes in human subjects, primates, andother domesticated and non-domesticated mammals. Upon determination thata patient or other subject would benefit from electrical stimulationfrom a device according to the invention, the clinician would firstimplant the device onto the spinal cord. The location may bepredetermined by imaging the spine and/or doing neurological studies,and then selecting a location that would be most likely to convey thedesired benefit.

For some purposes, the device may be supplied from the manufacturer in astandard size that can accommodate almost the full range of spinal cordanatomy variations encountered in patients. Alternatively, the devicecan be built in a plurality of different standard sizes, or may becustom manufactured for particular patients. In these circumstances, themethod of installing the device would further comprise the step ofdetermining appropriate dimensions of the patient's anatomy (such ascircumference or cross-sectional shape of the spinal cord and/or thespinal canal on the dorsal side, and/or dimensions of the vertebra towhich the device is to be secured.

Clinical Use

The device is implanted by conforming the arrayed electrodes to a regionof the spinal cord so that the electrodes directly contact the spinalcord; and then securing the device in place. Once fixed in place, itremains after surgical closure, and maintains the electrodes in contactwith the spinal cord, notwithstanding normal pulsation and mobility ofthe spinal cord, movement of the patient in ordinary daily activity, andmovements resulting mechanical such as might result if the patient slipsor falls. The affixing of the device, while robust, is preferablyreversible so that the device can later be removed or repositioned ifneeded, while causing minimal damage to the tissues.

Once implanted, the electrode array can be used for stimulating a spinalcord of a patient. The patient may be subject or susceptible to noxiousor deleterious nerve signals transmitted along the spinal cord, orotherwise requires treatment. An electrical stimulus is provided throughthe electrodes in the array directly to the spinal cord so as to inhibittransmission of such noxious or deleterious nerve signals.

The stimulus may be applied to inhibit sensation of pain, or to inhibitsymptoms or sensory input that is undesirable or disruptive to thepatient, either in the back itself, the extremities, or at anotherlocation wherein the pain is mediated at least in part by the spinalcord. Conditions suitable for treatment include back pain, leg pain,Parkinson's disease, spinal cord injury, Failed Back Surgery Syndrome,arthritic degeneration, phantom limb pain, numbness or palsy, orcongestive heart failure. The stimulus may be provided to the spinalcord by the device on a constitutive basis, in response to feedbackdata, or it may be subject to the patient's conscious control.

The treating clinician may select any electrical stimulus that iseffective in managing pain of a particular patient. The general objectis to induce refractoriness of the spinal cord to transmit noxious ordeleterious signals or synchronous depolarization events initiatedlocally. This can be adjusted empirically by determining neural activityand recording the symptoms experienced by the patient

Different patterns of stimulation may be effective depending on clinicalcircumstances. Under control of an appropriately programmedmicroprocessor or any other suitable type of controlled signalgenerator, electrodes in the array may all provide the same signalpattern, or individual or groups of electrodes may have their own signalpattern configured to work independently or in concert with signalpatterns of other electrodes in the array.

The electrical potential may vary at a regular frequency in a sinusoidalor square wave form. Alternatively, the wave form may be a more complexpattern, with pulses appearing at varying intervals and intensitiesaccording to a calculated or repetitive pattern. Such patterns comprisea pulse train generating substantially continuous activation of nerveswithin the spinal cord, and may incorporate irregular pulse intervals,irregular pulse amplitudes, a variety of wave forms (for example,monophasic, biphasic, rectangular, sinusoidal, as well as asymmetric orirregular wave forms), or any combination thereof. The potential maycreate what is essentially a broad band noise, varying at stochastic oressentially random intervals and intensity under the influence of asuitable computational algorithm or automated control program in amicroprocessor.

Depending on the objective of the treatment and the manner in which thetechnology is deployed, effective pulse repetition rates or frequenciesmay be at very low frequencies, or above 100 Hz (pulses per second), orat higher frequencies to cause stochastic depolarization, as describedbelow.

High Frequency Stimulation to Cause Stochastic Depolarization

High frequency stimulation of the spinal cord may benefit the patent byinducing a state of pseudospontaneous axon firing. Bundles of sensoryaxons are thought to fire randomly when not transmitting sensorystimulus. When a sensory stimulus is presented, a substantial proportionof the axons within a bundle or pathway will discharge in a synchronousfashion—firing axons potentials at about the same time. This results inthe sensory input being transmitted along the axons in the bundle, sothat the subject may experience the sensation. Stated differently, theabsence of sensation is coded by random timing of axon firing within abundle, whereas a sensory perception is coded by synchronous firing of apopulation of axons.

It is a hypothesis of this invention that patients with leg and backpain have bundles of axons spontaneously firing in a synchronous manner(or some other non-random fashion), instead of the normal random patternof firing. Electrical pulses will entrain axonal firing. A single pulsedelivered to a bundle of axons will cause them all to firesynchronously. If the time interval between each electrical shock in apulse train is longer than the refractory period of the axons in thebundle, each subsequent shock will also synchronously activate all ofthe axons, and a subject will experience a sensation. A low frequencyalternating current applied to the back (50 Hz) may be effective inreducing the sensation of pain, but the stimulation may generateneurological side effects such as paresthesias (tingling or numbness).

A high frequency electrical stimulus (say, about 5,000 Hz) has intervalspacing shorter than the refractory period of axons. An individual axoncannot fire again in response to a second shock until its membranepotential has recovered from the effects of the first shock, and thistakes time. Different axons have different refractory periods. Bydelivering electrical pulses at high frequency, the relative timing offiring by individual axons within the bundle of axons becomes nearlyrandom, with different axons become excitable again at different times.Applying high frequency pulses to the spinal cord can be used to restorea state of active quiescence in the sensory nerves passing through thecord.

Put another way, the spinal cord is stimulated so as to inhibit paintransmission by applying directly to the spinal cord an electricalstimulus that renders sensory neurons refractory to transmission ofsynchronous action potentials initiated within the spinal cord. Thisinhibits back pain from locally induced sensory input, and side effectssuch as paresthesia that may be induced in the course of localtreatment. The electrical stimulus is thought to promote stochasticdepolarization of sensory neurons within the spinal cord, thus inducinga state of neural quiescence.

To accomplish this, the electrical stimulus comprises a potential thatalternates at high frequency. Regardless of the way the potential mayvary over time, the frequency may be calculated by determining thenumber of positive-to-negative alterations per unit time. Effectivefrequency ranges depend on place of placement of the electrode array,the features of the array, the nature and health of the tissue where thearray is placed, and the objectives of treatment. The general object isto induce refractoriness of the spinal cord to transmit deleterioussignals or synchronous depolarization events initiated locally. This canbe adjusted empirically by determining neural activity and recording thesymptoms experienced by the patient.

Depending on the objective of the treatment and the manner in which thetechnology is deployed, effective pulse repetition rates or frequenciesmay be at or above 100 Hz (pulses per second), 200 Hz, 500 Hz, 2,000 Hz,or 5,000 Hz, a frequency of about 1,000 Hz, 4,000 Hz, or 10,000 Hz, or afrequency range of about 500 to 50,000 Hz, 1,000 to 9,000 Hz, 3,000 to8,000 Hz, 2,000 to 20,000 Hz, or 5,000 to 15,000 Hz.

Treating back pain according to the invention may comprise administeringan effective electronic stimulus to the spinal cord, monitoringtransmission of synchronous action potential through the spinal cord orinferring the same, and then adjusting the electrical stimulus so as tofurther inhibit transmission through the spinal cord of synchronousaction potentials. The electrical stimulus may be adjusted in frequencyor other waveform parameters and manner of application so as to minimizeside effects such as paresthesia, and to minimize impact on transmissionof essential neurological faction, including motor neuron activity, andnerves involved in proprioception and kinesthesia. Optionally, theclinician or the user may be provided with an input or control means toselect the pattern, adjust the frequency, and adjust the intensity inaccordance with the perceived symptoms.

EXAMPLES Example 1: Optimization of the Electrode Array Structure UsingThoracic Spine Imaging Data

An important dimensional parameter for a spinal cord stimulator array isthe arc length that it subtends over the dorsal surface of the spinalcord. There are conflicting design goals. One is to make that span aslong as possible, in order to maximize the number of electrodes andhence the stimulus-pattern coverage of the underlying dorsal columns.However, another is to ensure that the membrane does not make mechanicalcontact with the dorsal rootlets.

In this study, structural dimensions of the spinal cord at the level ofthe 4^(th) through 10^(th) thoracic vertebrae were investigated (theregion in which the array would be placed when treating back and legpain). The arc length “S” between dorsal-root entry zones (DREZ) wascalculated in two ways from magnetic resonance (MR) images of thethoracic spine for 50 patients seen at the University of Iowa Hospitalsand Clinics.

One axial and sagittal image from each patient was selected foranalysis. The available images covered the range T4 through T10 fromhigh-resolution MR scans of both male (ages 17 to 77 years) and female(ages 20 to 84 years) patients. The imaging studies had been ordered byclinicians to rule out pathological processes affecting the spine, andin all cases studied no pathological abnormalities were noted. All ofthe subjects were imaged in the supine position in a straight posturewithout any bending or flexing of the legs, hips or spine. Of theselected slices, 70% (n=35) were at either T7 or T8 which will be thepreferred location for positioning the array in most patients. Theremaining images were distributed above and below that zone to helpinsure a representative assessment.

With reference to FIG. 12A, the bi-lateral locations of the dorsal rootentry zones, P₁ and P₃, were identified by neurosurgeons on each of the50 axial slices, and the linear separation, A, between them was measuredrelative to the calibration scale bar on each image. The distance, B,between the center of that line and the dorsal-most point of the spinalcord, P₂, was also measured, as were the maximum sagittal and coronaldiameters of the spinal cord (i.e., the minor and major axis diameters,respectively). The resulting data were then archived for subsequentanalysis, with the primary goal being to determine the peripheral arclength, S, connecting the points P₁, P₂ and P₃.

The cross section of the spinal cord in the thoracic region is roughlyoval in shape, but with an irregular circumference that departs from anellipse. S was estimated by computing the hypotenuse to the triangleformed by A/2 and B: S_(H)≈2·[(A/2)²+B²]^(1/2). Because the actual arcrises just above that hypotenuse, S_(H) slightly underestimates S.Alternatively, with reference to FIG. 12B, S was estimated by way of thecircular arc length: S_(R)≈r·θ, where r is the estimated value of radialdistance between the geometric center of the spinal cord and the pointsP₁ and P₃, and θ is the angular separation between those lines. Becausethe actual arc lies below the circumscribing path of S_(R), thiscalculation slightly overestimates S.

Thus, S_(H)<S<S_(R). Measurements to confirm this and establish the mostlikely value of S within that range can then be made directly on amagnified view of an axial image, using a flexible rule and theappropriate scaling factor to determine the distance along the span. Theestimate of S, as determined across the entire patient population, canbe used as a design guide for the membrane length of the patch. Thevalue of “r” determined from the measured major and minor axisdiameters, can then be used to establish the radius of curvature for thearray membrane.

The measured values of A and B in mm were as follows: All patients(n=50), 5.8±0.8, 1.5±0.4; males (n=34), 5.9±0.8, 1.5±0.4; females(n=16), 5.5±0.7, 1.5±0.4 The relative uncertainties (standarddeviation+mean) in the values of A and B across all patients was 14% and27%, respectively. The value of A across all male patients wasapproximately 2% larger than the mean for all patients, and that for thefemale patients was approximately 5% smaller. The calculated value ofS_(H)=6.5±1.2 mm. The difference between the largest mean value of S_(H)(8.8 mm) and the smallest (5.1 mm) was 3.7 mm—approximately three timesthe size of the standard deviation (1.2 mm), indicating that this is adimension of the neuroanatomy in which significant outliers occur.

The sagittal and coronal diameters of the spinal cord in each of the 50axial images were 6.2±0.6 mm and 8.3±0.8 mm respectively. Thus, the meanradius and quadratured-sum uncertainty of the spinal cord is r=3.6±0.5mm. Upon review of other results, it seemed most conservative to taker=4.1 mm to be the working value of the mean radius. Using θ≈95°,S_(R)=6.8±1.0 mm, where the uncertainty is given by the quadratured sumof those measured for r and estimated for θ.

The calculated values of S_(H) and S_(R) were compared against physicalmeasurements made with a flexible rule laid carefully along the dorsalarc pathway of images expanded 3-fold. The length of the dorsal arc spanbetween the rootlet entry zones was estimated to be S=6.7±1.0 mm.

Thus, if the width of an electrode array was were 6.7−1.0=5.7 mm, thenit would be a good fit to the spinal cords of at least 68% (1σ) of thepatients receiving the implant. The problem would come with the outliersat the high and low ends of the distribution of arc lengths. Providingthree different widths of 8 mm, 6 mm and 4 mm would be suitable forsubstantially all the adult population. The largest size device wouldhave additional electrode contacts and leads. Alternatively, customarrays could be fabricated for individual patients usingpatient-specific arc length measurements.

The mean radius of the spinal cord across all patients was r=3.6±0.5 mm.A nominal mean value of r=4.1 mm would be suitable for curvature of thearray. Opting for a slightly larger radius of curvature reduces the riskof spinal cord compression that might arise from too small a sizing.

Example 2: MR-Based Measurement of Spinal Cord Motion During Flexion ofthe Spine: Implications for Intradural Spinal Cord Stimulator Systems

For purposes of this study, a 1.5 T Magnetom Espree® magnet (Siemens,Erlangen, Germany) was used. Informed consent was obtained from healthyvolunteers ranging in age from 23 to 58. Each volunteer was first imagedin a supine neutral position and then imaged in a maximal attainableflexed position.

To obtain the maximal flexion of the spine, patients were given threebasic positioning instructions. The first was to rotate their pelvisbackwards towards the gantry as far as possible to remove the lumbarlordosis and straighten the lumbar spine. The second was to curl theirupper back, neck and head forward so that their shoulders were as closeto their knees as possible. The third instruction was then to tuck theirchin down as close to their chest as possible. While attaining thisflexed position in the bore, a variety of foam wedges and pillows wereused for added support so that the patient could remain as still aspossible during image acquisition. Maximal flexion was limited byvolunteer flexibility in 14 of the patients. In only two patients wasflexion limited by MR bore size.

Each volunteer had a vitamin E capsule taped to their midline lowerthoracic spine for help in level localization. A sagittal HASTE sequencewas performed initially as a localizer both for vertebral level countingand identification of a more focal field of view centered over theregion of lowest thoracic spinal nerves and the conus medullaris. Toacquire anatomic images with enough resolution to accurately measureintervals between spinal nerve dorsal root entry zones, a CISS sequencewas selected for its high spatial resolution. Although this is a highlyT2-weighted sequence, acquisition time still required 2 minutes and 5seconds. This length of time initially caused too much motiondegradation during flexed imaging to make accurate measurements. The useof the pillows and foam wedges provided just enough support forvolunteers to remain still for the required two minute duration. Allneutral and flexed sequences were obtained using TR=4.35 ms, TE=2.18 ms,slice thickness=0.8 mm, matrix size=192×192, one acquisition peraverage, 192 phase encoding steps, field of view=200 mm, and a 70 degreeflip angle.

FIG. 13B shows an example of a coronal image on which the relevantanatomical features are identified. Imaging was obtained in the coronalplane. Three-dimensional multiplanar reconstruction software was used ona Carestream PACS station to aid in measurement. The T10 and T11 nerveroots were identified. A cranial caudal measurement was made in a planeparallel to the spinal canal between the dorsal-root entry zones (DREZ)of T10 and T11. (The exact position of the entry zones was confirmed byassessing sequential axial images to identify the most cranial aspect ofthe nerve originating from the spinal cord.) As shown in FIG. 13A, thedifference between this measurement on the neutral and flexed images isa measure of spinal cord contraction/expansion along the rostral-caudalaxis. Next, a cranial caudal measurement was made from the DREZ of theT10 nerve root along the same plane as the prior measurement, to thelevel of a plane orthogonal to the spinal canal at the level of theinferior T10 pedicles. The latter were selected as a reference point ofthe bony canal inside of which the spinal cord moves. The differencebetween these measurements represents cord movement within the bonycanal.

A cranial caudal measurement of the change in conus tip position wasmade. To accurately accomplish this, the position of the conus tip wasfirst identified on the neutral images with reference to a landmarkwithin the bony spinal canal at the same cranial-caudal level. Thislandmark was then identified on flexed imaging and a cranial caudalmeasurement was made from that level to the level of the new conusposition. This represents movement of spinal cord within the canal.

Results were as follows. The spinal cord should move rostrally duringflexion and should lie in its most caudal location when the patient isin the neutral position. The measured change in the pedicle-to-spinalcord DREZ distance across all patients between the neutral and flexionpositions ranged from 1.9 mm to 18.0 mm, with a mean and standarddeviation of 8.5±6.0 mm. The inter-DREZ distance across all patientsbetween the neutral and flexion positions ranged from −2.0 mm to +6.7mm, with a mean and standard deviation of 3.5±2.6 mm. The mean andstandard deviation for the rostral-caudal conus movement was found to be6.4±4.1 mm within an overall range of 1.1 to 11.4 mm. The fractionalvariations in these findings (standard deviation÷mean) are very large,71%, 74% and 64% respectively. This reflects the wide variability in thecapacity of individual subjects to maximally flex the spine, as well aspossible inter-subject variability in spinal cord mechanicalcharacteristics. These findings highlight the need for the device toaccommodate larger patient-to-patient variations in spinal cord dynamicmovement properties.

The ratio of the spinal cord's mean stretch-to-mean axial movement overa full flexion cycle was 3.5 mm/8.5 mm≈40%. On average across allpatients, it required 1 mm of net axial displacement of the cord tostretch it 0.4 mm in length. A spinal cord stimulator device shouldaccommodate a total rostral-caudal motion of up to ˜2 cm of thecord/membrane relative to the fixation point, i.e., 1 cm rostral and 1cm caudal from the neutral position.

A prototype device of the type shown in FIG. 14 (loop area z 160 mm²)was used to test the available range of motion. It comprises an analogueelectrode bearing portion 11 without electrodes, a spring portion 12,and a securing strap 41. The prototype device was placed on acustom-designed silicone surrogate spinal cord specimen that waspositioned inside an anthropomorphic spinal canal phantom (not shown).The device was able to accommodate this level of motion without lift-offof either end of the membrane when the surrogate reached the 1 cmrostral and caudal extremes of displacement.

Since there were large variations (70%) in the magnitude of that motionfrom patient to patient, there will be a spectrum of spinal cord strainsassociated with flexion-driven motion of the cord. Having suitable axialcompliance within the electrode bearing portion of the device willreduce the risk of potential irritation of the pial surface in patientswhere the intraparenchymal strains are large. In patients with smalllevels of strain, there would be little relative motion between cord andthe array, meaning that there would be small risk of any skiddingbetween them. The net axial travel of the spinal cord relative to thefixation point is within the range that can be accommodated withoutlift-off of the electrode bearing portion of the device.

Each and every publication and patent document cited in this disclosureis hereby incorporated herein by reference in its entirety for allpurposes to the same extent as if each such publication or document wasspecifically and individually indicated to be incorporated herein byreference.

While the invention has been described with reference to the specificembodiments, changes can be made and equivalents can be substituted toadapt to a particular context or intended use, thereby achievingbenefits of the invention without departing from the scope of what isclaimed.

The invention claimed is:
 1. A method for stimulating a spinal cord thatis prone to transmit deleterious nerve signals in a subject, the methodcomprising: implanting an electrode assembly into the subject at aposition between the pial surface of the spinal cord and the dura materthat surrounds the spinal cord, wherefrom the assembly does not obstructflow of cerebrospinal fluid (CSF) around the spinal cord; implanting asignal generator into the subject outside of the dura; electricallyconnecting leads from the electrode assembly through the dura to thesignal generator; securing the electrode assembly to the dura so as tomaintain the assembly at said position; and then applying to the spinalcord by way of the electrode assembly an electrical stimulus thatinhibits transmission of the deleterious nerve signals.
 2. The method ofclaim 1, wherein the implanting includes securing the electrode assemblyto the dura of the spinal cord by a water-tight dura-traversing leadfitting.
 3. The method of claim 1, wherein the implanting includessecuring the electrode assembly wherefrom electrodes in the electrodeassembly are maintained in direct contact with a region of the spinalcord unaffected by movement of the subject.
 4. The method of claim 1,wherein the electrode assembly comprises at least 10 electrodes arrayedon a pliable backing.
 5. The method of claim 1, wherein The electricalstimulus alternates at a frequency between 2,000 and 20,000 Hz.
 6. Amethod for inhibiting transmission of deleterious nerve signals throughthe spinal cord of a subject in need thereof, wherein the subject hasbeen implanted with a medical device that comprises: (1) an electrodeassembly, implanted between the pial surface and the dura mater of thespinal cord and secured to the dura without tethering the spinal cord tothe dura; (2) a signal generator, implanted outside the dura; and (3)leads that electrically connect the signal generator through the dura tothe electrode assembly; wherein the method comprises applying to thespinal cord by way of the electrode assembly an electrical stimulus,thereby inhibiting transmission of the deleterious nerve signals throughthe subject's spinal cord.
 7. The method of claim 6, wherein theelectrode assembly has been secured to the dura of the subject by awater-tight dura-traversing lead fitting.
 8. The method of claim 6,wherein the electrical stimulus alternates at a frequency that causesstochastic depolarization of the spinal cord, thereby inhibitingtransmission of synchronous action potentials initiated within thespinal cord.
 9. The method of claim 6, further comprising adjusting thefrequency of the electrical stimulus that is being applied to the spinalcord of the subject.
 10. The method of claim 6, wherein the electricalstimulus alternates at a frequency at or above 500 Hz.
 11. The method ofclaim 6, wherein of the electrical stimulus alternates at a frequencybetween 2,000 and 20,000 Hz.
 12. The method of claim 6, wherein theelectrical stimulus varies in a non-uniform pattern or at stochasticintervals.
 13. A method of treating back pain in a subject, comprisinginhibiting transmission of deleterious nerve signals through the spinalcord of the subject according to the method of claim
 6. 14. An assemblyof components for a medical device that is configured for stimulating asubject's spinal cord, the assembly comprising: an electrode assemblythat has a sufficiently thin profile to be implanted at a positionbetween the pial surface of the spinal cord and the dura mater withoutobstructing flow of cerebrospinal fluid (CSF) around the spinal cord,wherein the electrode assembly is configured to be secured to the duraso as to maintain the assembly at said position; a plurality ofelectrodes positioned on the electrode assembly that are directedtowards the surface of the spinal cord when the electrode assembly isthus implanted and secured; electrical leads passing from the pluralityof electrodes of the electrode assembly that are electricallyconnectable through the dura to a signal generator located outside thedura when the electrode assembly is thus implanted and secured; and saidsignal generator, implantable in the subject at a location outside thedura of the spinal cord wherefrom it is connectable by way of theelectrical leads to the electrodes of the electrode assembly thusimplanted and secured; wherein the signal generator is configured totransmit electrical signals through the leads to the plurality ofelectrodes of the electrode assembly, and thereby to apply to the spinalcord of the subject an electrical stimulus to stimulate the subject'sspinal cord.
 15. An assembly of components according to claim 14,wherein the electrical stimulus alternates at a frequency at or above200 Hz.
 16. An assembly of components according to claim 14, wherein theelectrical stimulus alternates at a frequency between 2,000 and 20,000Hz.
 17. An assembly of components according to claim 14, wherein thefrequency of the electrical stimulus that is applied to the spinal cordof the subject is adjustable.
 18. An assembly of components according toclaim 14, wherein the electrode assembly comprises at least 10electrodes arrayed on a pliable backing.
 19. An assembly of componentsaccording to claim 14, further comprising a lead fitting that isconfigured to secure the electrode assembly to the dura to form awater-tight closure, through which the electrical leads from theelectrode array inside the dura are connectable to the signal generatoroutside the dura.
 20. An assembly of components according to accordingto claim 14, wherein the electrical stimulus is adjustable in responseto pain experienced by the subject so as to further inhibit transmissionthrough the spinal cord of synchronous action potentials.